hBN Flakes Research Articles

Integration of multi-layer black phosphorus into photoconductive antennas for THz emission

We report the fabrication, characterization, and modeling of photoconductive antennas (PCAs) using 40 nm thin-film flakes of black phosphorus (BP) as the photoconductor and hexagonal boron nitride (hBN) as a capping layer to prevent oxidation of BP. Dipole antennas were fabricated on oxidized high-resistivity Si substrates, and BP and hBN flakes were picked up and transferred onto the antenna inside a nitrogen glovebox. The transfer matrix technique was used to optimize the thickness of BP and hBN for maximum absorption. BP flakes were aligned with the armchair axis along the anode–cathode gap of the antenna, with crystal orientation measured using reflection anisotropy. Photocurrent imaging under illumination with 100 fs pulses at 780 and 1560 nm showed a bias-dependent maximum photocurrent localized to the antenna gap with a peak photoconductivity of 1 (2) S/cm in the linear regime of bias for excitation at 780 (1560) nm. Photocurrent saturation in bias (pump fluence) occurred at approximately 1 V (0.25mJ/cm2). Device performance was modeled numerically by solving Maxwell’s equations and the drift–diffusion equation to obtain the photocurrent density in response to pulsed laser excitation, which was largely in qualitative agreement with the experimental observations. THz output computed from surface current density suggests that BP THz PCA performance is at least comparable to more traditional devices based on low-temperature-grown GaAs. These devices represent a step toward high-performance THz photoconductive antennas using BP.

Thermomechanical behaviour of hexagonal boron nitride at elevated temperatures

Hexagonal boron nitride (hBN) is attractive for applications at elevated temperatures due to its excellent thermal properties. It is thus of significant importance the knowledge of its thermomechanical behaviour and related structural changes at high temperatures. Herein, we heat up hBN of thickness ranging from one to four layers and monitor its mechanical response with Raman spectroscopy upon cooling to room temperature under constrained conditions. The thermal expansion coefficient (TEC) of hBN is thus estimated and is found to be negative in all cases and to decrease with the increase of flake thickness. The strain induced to the hBN flakes due to the thermal shrinkage is estimated to be in the range of ∼0.065–0.15%. Topographic images taken using atomic force microscopy reveal structural changes such as wrinkling formation as a result of the induced compressive strain upon cooling. The suspended hBN fails almost instantaneously under compressive load. The supported parts sustain higher compressive strain prior to failure and form localized wrinkles with much smaller wavelength as compared to the suspended hBN. The wrinkles formed in the supported section are qualitatively similar to those observed in graphene, reflecting the hexagonal lattice symmetry of these crystals. Finally, knowing the biaxial shift of the Raman G-peak with strain, we can herein calculate the maximum compression strain the supported hBN can support as a function of number of layers (from 1 to 4).

Van der Waals Bound Organic/2D Insulator Hybrid Structures: Epitaxial Growth of Acene Films on hBN(001) and the Influence of Surface Defects.

Combining 2D materials with functional molecular films enables the fabrication of van der Waals bound organic/inorganic hybrids that are of interest for future device architectures. Recently, the 2D dielectric hexagonal boron nitride (hBN) has received particular attention since exfoliation allows the preparation of crystalline layers which have been utilized as ultrathin dielectrics in electronic devices. Here, we have studied the formation and structure of molecular films of the prototypical organic semiconductors pentacene (PEN) and perfluoropentacene (PFP) on hBN. Special attention was paid to the influence of substrate surface defects on the film formation by comparing molecular films that were grown on hBN substrates of various quality, including single crystals (representing the most ideal surface), briefly ion bombarded substrates, and exfoliated flakes. While X-ray diffraction (XRD) yields precise information about the crystalline structure of films grown on (large) single crystals, it is hardly applicable to analyze the films formed on exfoliated flakes because of their small size. Here, we demonstrate that in the case of flakes detailed structural analyses of the molecular films are possible by combining atomic force microscopy (AFM) with microspot UV/vis spectroscopy and optical polarization microscopy. On well-ordered hBN single crystal surfaces both acenes form very smooth and epitaxial crystalline films where molecules adopt a recumbent orientation (even in 100 nm thick films). By contrast, both materials adopt an upright molecular orientation and different polymorphs on defective hBN surfaces and reveal distinctly different film morphologies. On exfoliated flakes, PFP shows a film structure similar to that on the hBN single crystals, while PEN films exhibit a structure as on defective hBN substrates. In addition, a pronounced decoration of defect steps, which are probably created by the exfoliation process, was observed for PEN leading to the formation of tall and extended fibers where molecules adopt a recumbent orientation. The present study reveals different robustness in film growth on exfoliated hBN flakes for various molecules, which has to be considered in their device integration, especially with regard to their optoelectronic properties such as light absorption or charge transport, which depend critically on the molecular orientation and crystalline order.

A hybrid structure light-emitting device based on a CsPbBr3 nanoplate and two-dimensional materials

High-crystalline halide perovskite nanostructures [such as nanowires and nanoplates (NPs)] provide good potential in realizing nanoscale solid light sources for on-chip optical communication, high-density storage, and life science applications. However, it remains a great challenge to fabricate nanoscale perovskite light-emitting devices using traditional fabrication methods because the perovskite nanomaterials will be dissolved in polar solvents. Developing new device configurations to enhance radiative recombination efficiency as well as device stability is one of the most important research topics in nanoscale perovskite light-emitting devices. Here, we demonstrate nanoscale perovskite electroluminescence (EL) using a single-crystalline CsPbBr3 NP as the active layer. The device is based on a hybrid capacitance structure, where an underlying few-layer graphene (FLG) electrode, a single-crystalline CsPbBr3 NP, a thin hexagonal boron nitride (hBN) flake, and another FLG top electrode are stacking in sequence, forming a van der Waals heterostructure. A strong EL emission peak with a narrow linewidth (∼1.09 nm) is observed at 2 K. Alternating current voltage/frequency-dependent EL spectra are studied in detail. We attribute the superior EL behavior of the as-fabricated nanoscale perovskite light-emitting devices to (1) the high-quality single-crystalline CsPbBr3 NPs synthesized, (2) the hBN encapsulation, which enhances the device stability by providing a large heat dissipation pathway for CsPbBr3 NP and protecting it from the polar solvents, (3) the capacitance structure, which facilitates the injection of both electrons and holes. Our work demonstrates a method to construct nanoscale perovskite (with well-defined geometry) light sources, providing an opportunity for realizing a nanoscale electrically driven perovskite laser.

Visibility of hexagonal boron nitride on transparent substrates

The high transmittance and low reflectance of monolayer hexagonal boron nitride (hBN) lead to its invisibility under white-light, causing serious troubles in the search, transfer, and fabrication of 2D material devices. In this work, we demonstrate enhancing the contrast of hBN on a transparent substrate by simulation and experimental observation, where the highest contrast is obtained by using a polymer-based interfacial layer on a polydimethylsiloxane (PDMS) substrate. The simulation result reveals that the contrast under short wavelength light is higher than that under long wavelength. To confirm this, the red-green-blue components are extracted from the optical color image. The blue component image shows an hBN flake clearly on the substrate, while the hBN flake fades on the green and red components. Moreover, the contrast on transparent substrates have only positive value, while opaque substrates cause both negative and positive contrast depending on the thickness of the interfacial layer. Thus, the high contrast (∼4.5%) of hBN on the PDMS substrate enables us to observe mono- and few-layer hBN flakes under white-light illumination by an optical microscope.

Triplet Excitation and Electroluminescence from a Supramolecular Monolayer Embedded in a Boron Nitride Tunnel Barrier.

We show that ordered monolayers of organic molecules stabilized by hydrogen bonding on the surface of exfoliated few-layer hexagonal boron nitride (hBN) flakes may be incorporated into van der Waals heterostructures with integral few-layer graphene contacts forming a molecular/two-dimensional hybrid tunneling diode. Electrons can tunnel through the hBN/molecular barrier under an applied voltage VSD, and we observe molecular electroluminescence from an excited singlet state with an emitted photon energy hν > eVSD, indicating upconversion by energies up to ∼1 eV. We show that tunneling electrons excite embedded molecules into singlet states in a two-step process via an intermediate triplet state through inelastic scattering and also observe direct emission from the triplet state. These heterostructures provide a solid-state device in which spin-triplet states, which cannot be generated by optical transitions, can be controllably excited and provide a new route to investigate the physics, chemistry, and quantum spin-based applications of triplet generation, emission, and molecular photon upconversion.

Second-harmonic generation in multilayer hexagonal boron nitride flakes

In this Letter, we report the second-harmonic generation (SHG) from thick hexagonal boron nitride (hBN) flakes with approximately 109-111 layers. The resulting effective second-order susceptibility is similar to previously reported few-layer experiments. This confirms that thick hBN flakes can serve as a platform for nonlinear optics, which is useful because thick flakes are easy to exfoliate while retaining a large size. We also show spatial second-harmonic maps revealing that SHG remains a useful tool for the characterization of the layer structure, even in the case of a large number of layers.

High optical quality of MoS2 monolayers grown by chemical vapor deposition

Chemical vapor deposition (CVD) allows growing transition metal dichalcogenides (TMDs) over large surface areas on inexpensive substrates. In this work, we correlate the structure of CVD grown MoS2 monolayers (MLs) on SiO2/Si wafers studied by high-resolution transmission electron microscopy (HRTEM) with the optical quality revealed in optical emission and absorption from cryogenic to ambient temperatures. We determine a defect concentration of the order of 1013 cm−2 for our samples with HRTEM. To have access to the intrinsic optical quality of the MLs, we remove the MLs from the SiO2 growth substrate and encapsulate them in hBN flakes with low defect density, to reduce the detrimental impact of dielectric disorder. We show optical transition linewidth of 5 meV at low temperature (T = 4 K) for the free excitons in emission and absorption. This is comparable to the best ML samples obtained by mechanical exfoliation of bulk material. The CVD grown MoS2 ML photoluminescence is dominated by free excitons and not defects even at low temperature. High optical quality of the samples is further confirmed by the observation of excited exciton states of the Rydberg series. We optically generate valley coherence and valley polarization in our CVD grown MoS2 layers, showing the possibility for studying spin and valley physics in CVD samples of large surface area.

Momentum-forbidden dark excitons in hBN-encapsulated monolayer MoS2

Encapsulation by hexagonal boron nitride (hBN) has been widely used to address intrinsic properties of two-dimensional (2D) materials. The hBN encapsulation, however, can alter properties of 2D materials through interlayer orbital hybridization. In this paper, we present measurements of temperature dependence of photoluminescence intensity from monolayer MoS2 encapsulated by hBN flakes. The obtained temperature dependence shows an opposite trend to that of previously observed in a monolayer MoS2 on a SiO2 substrate. This is caused by the existence of stable momentum-forbidden dark excitons in the hBN-encapsulated MoS2. Ab-initio band-structure calculations have shown that orbital hybridization between MoS2 and hBN leads to upward shift of Γ-valley of MoS2, which results in lowering of energy of the momentum-forbidden dark excitons. This work shows an important implication that the hBN-encapsulated structures used to address intrinsic properties of two-dimensional crystals can alter basic properties of encapsulated materials.

Thermal properties of multilayer graphene and hBN reinforced copper matrix composites

The unique properties of graphene make it a very attractive application, although there are still no commercial products in which graphene would play a key role. Good thermal conductivity is undoubtedly one of the attributes which can be easily used both in materials involving large monoatomic layers, that are very difficult to obtain, as well as multilayer graphene flakes, which have been commercially available on the market for several years. The article presents the results of tests on the characteristic thermal properties of composites with the addition of 2–15% of multilayer graphene (MLG) crystals. The motivation of the study was literature reports showing the possibility of increasing the thermal conductivity of composites with MLG participation in the copper matrix. Since the production of composites with increased properties is associated with obtaining a strong orientation of the flakes in the structure, composites with hBN flakes exhibiting significantly worse but also directional thermal properties were produced for comparison. The paper showed a strong influence of flake morphology on the possibility of creating a directional structure. The obtained Cu/MLG composites with the addition of only 2% MLG were characterized by an increase in the thermal conductivity coefficient of about 30% in relation to sinters without the participation of MLG.

Effects of Hexagonal Boron Nitride Encapsulation on the Electronic Structure of Few-Layer MoS2

The hexagonal boron nitride (hBN) encapsulation has been widely used in the electronics applications of 2D materials to improve device performance by protecting 2D materials against contamination and degradation. It is often assumed that hBN layers as a dielectric would not affect the electronic structure of encapsulated 2D materials. Here we studied few-layer MoS$_2$ encapsulated in hBN flakes by using a combination of theoretical and experimental Raman spectroscopy. We found that after the encapsulation the out-of-plane A$_{1g}$ mode is upshifted, while the in-plane E$_{2g}^1$ mode is downshifted. The measured downshift of the E$_{2g}^1$ mode does not decrease with increasing the thickness of MoS$_2$, which can be attributed to tensile strains in bilayer and trilayer MoS$_2$ caused by the typical experimental process of the hBN encapsulation. We estimated the strain magnitude and found that the induced strain may cause the K-Q crossover in the conduction band of few-layer MoS$_2$, so greatly modifies its electronic properties as an n-type semiconductor. Our study suggests that the hBN encapsulation should be used with caution, as it may affect the electronic properties of encapsulated few-layer 2D materials.

Nanospot angle-resolved photoemission study of Bernal-stacked bilayer graphene on hexagonal boron nitride: Band structure and local variation of lattice alignment

Hexagonal boron nitride (hBN) is the supporting substrate of choice for two-dimensional material devices because it is atomically flat and chemically inert. However, due to the small size of mechanically exfoliated hBN flakes, electronic structure studies of 2D materials supported by hBN using angle-resolved photoemission spectroscopy (ARPES) are challenging. Here we investigate the electronic band structure of a Bernal-stacked bilayer graphene sheet on a hexagonal boron nitride (BLG/hBN) flake using nanospot ARPES (nanoARPES). By fitting high-resolution energy vs. momentum electronic band spectra, we extract the tight-binding parameters for BLG on hBN. In addition, we reveal spatial variations of the alignment angle between BLG and hBN lattices via inhomogeneity of the electronic bands near the Fermi level. We confirmed these findings by scanning tunneling microscopy measurements obtained on the same device. Our results from spatially resolved nanoARPES measurements of BLG/hBN heterostructures are instrumental for understanding experiments that utilize spatially averaging techniques such as electronic transport and optical spectroscopy.

Competition between stimulated Brillouin scattering and two-photon absorption in dispersed boron nitride.

The design of transparent optical materials with stimulated Brillouin scattering (SBS) suppression is a topic of current interest. We measured two-photon absorption (2PA) cross-section σ2PA and Brillouin gain factor gB of a suspension of hexagonal boron nitride (hBN) in N-methyl-2-pyrrolidone at the second harmonic of a Nd:YAG laser. SBS exhibits a significant quenching with hBN concentration, like previously observed in graphene suspension. The melting of hBN flakes due to a large 2PA and the related changes in the acoustic damping coefficient explain the quenching mechanism.

Low temperature self-densification of high strength bulk hexagonal boron nitride

Hexagonal boron nitride (hBN) ceramics are expected to have wide applications at high temperatures as both a structural and functional material. However, because of its flake structure and general inertness, it is currently impossible to sinter hBN powder to a dense bulk (with a relative density of above 96%) even at 2000 °C. Here, we report dense bulk hBN with 97.6% theoretical density achieved at a lower preparation temperature (1700 °C) via a self-densifying mechanism without sintering additives. During the sintering process, cubic boron nitride particles incorporated into the hBN flake powders transform into BN onions with a significant volume increase, thus filling in voids among the hBN flakes and highly densifying the hBN bulks. The resulting dense hBN ceramics possess 2–3 times the strength of traditional hBN ceramics. This phase-transition-induced volume expansion strategy could lead to dense sintered compacts with high performance in other ceramic systems.

Superlubricity enabled dry transfer of non-encapsulated graphene**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0302001) and the National Natural Science Foundation of China (Grant Nos. 11574204 and 11774224).

Transferring high-quality exfoliated graphene flakes onto different substrates while keeping the graphene free of polymer residues is of great importance, but at the same time very challenging. Currently, the only feasible way is the so-called all-dry “pick-and-lift” method, in which a hexagonal boron nitride (hBN) flake is employed to serve as a stamp to pick up graphene from one substrate and to lift it down onto another substrate. The transferred graphene samples, however, are always covered or encapsulated by hBN flakes, which leads to difficulties in further characterizations. Here, we report an improved “pick-and-lift” method, which allows ultra-clean graphene flakes to be transferred onto a variety of substrates without hBN coverage. Basically, by exploiting the superlubricity at the graphene/hBN stack interface, we are able to remove the top-layer hBN stamp by applying a tangential force and expose the underneath graphene.

Probing the nanoscale origin of strain and doping in graphene-hBN heterostructures

We use confocal Raman microscopy and a recently proposed vector analysis scheme to investigate the nanoscale origin of strain and carrier concentration in exfoliated graphene-hexagonal boron nitride (hBN) heterostructures on silicon dioxide (SiO2). Two types of heterostructures are studied: graphene on SiO2 partially covered by hBN, and graphene fully encapsulated between two hBN flakes. We extend the vector analysis method to produce separated spatial maps of the strain and doping variation across the heterostructures. This allows us to visualise and directly quantify the much-speculated effect of the environment on carrier concentration in graphene. Moreover, we demonstrate that variations in strain and carrier concentration in graphene arise from nanoscale features of the heterostructures such as fractures, folds and bubbles trapped between layers. For bubbles in hBN-encapsulated graphene, hydrostatic strain is shown to be greatest at bubble centres, whereas the maximum carrier concentration is localised at bubble edges. Raman spectroscopy is shown to be a non-invasive tool for probing strain and doping in graphene, which could prove useful for engineering of two-dimensional devices.

Waterproof Perovskite-Hexagonal Boron Nitride Hybrid Nanolasers with Low Lasing Thresholds and High Operating Temperature

Solid-state perovskites have recently emerged as promising coherent light sources, due to their efficient gain. While the continuous-wave (CW) optically pumped perovskite laser has been achieved at low temperatures, the final frontier of an electrical perovskite-based laser diode remains challenging due to the heat management and intrinsic instability of perovskite materials. Here, we demonstrate waterproof perovskite-hexagonal boron nitride (hBN) hybrid nanolasers with low lasing thresholds and high operating temperature. After capping with the hBN flake, which possesses superb and anisotropic thermal conductivity, heat dissipation of the perovskite nanolaser is accelerated and an overheated hot spot is avoided. This results in the significant reduction of lasing thresholds (17.05% to 60.15%) in 16 measured samples and clear lasing behavior under a temperature as high as 75.6 °C. Moreover, hBN with high environmental stability can effectively protect the perovskite from the polar solvents. The hBN encapsulated CsPbI3 nanolaser can incessantly lase in water for an hour, and the lasing behavior can be retained even after 24 h of immersion in water. The reduction of lasing threshold, improved heat removal, and higher temperature tolerance of the hybrid structure nanolaser marks a major step toward a CW-pumped perovskite laser at room temperature, while also allowing perovskites to be integrated into high power density optoelectronic devices and future electrically driven lasers. In addition, as the sandwiching of perovskites with hBN can withstand polar solvents, this method may serve as a reliable method for both the future fabrication of perovskite-based devices and sensor-based applications in solvent systems.

Intrinsic excitonic emission and valley Zeeman splitting in epitaxial MS2 (M = Mo and W) monolayers on hexagonal boron nitride

Two-dimensional (2D) semiconductors, represented by 2D transition metal dichalcogenides (TMDs), exhibit rich valley physics due to strong spin-orbit/spin-valley coupling. The most common way to probe such 2D systems is to utilize optical methods, which can monitor light emissions from various excitonic states and further help in understanding the physics behind such phenomena. Therefore, 2D TMDs with good optical quality are in great demand. Here, we report a method to directly grow epitaxial WS2 and MoS2 monolayers on hexagonal boron nitride (hBN) flakes with a high yield and high optical quality; these monolayers show better intrinsic light emission features than exfoliated monolayers from natural crystals. For the first time, the valley Zeeman splitting of WS2 and MoS2 monolayers on hBN has been visualized and systematically investigated. This study paves a new way to produce high optical quality WS2 and MoS2 monolayers and to exploit their intrinsic properties in a multitude of applications.

Anomalous Pressure Characteristics of Defects in Hexagonal Boron Nitride Flakes.

Research on hexagonal boron nitride (hBN) has been intensified recently due to the application of hBN as a promising system of single-photon emitters. To date, the single photon origin remains under debate even though many experiments and theoretical calculations have been performed. We have measured the pressure-dependent photoluminescence (PL) spectra of hBN flakes at low temperatures by using a diamond anvil cell device. The absolute values of the pressure coefficients of discrete PL emission lines are all below 15 meV/GPa, which is much lower than the pressure-induced 36 meV/GPa redshift rate of the hBN bandgap. These PL emission lines originate from atom-like localized defect levels confined within the bandgap of the hBN flakes. Interestingly, the experimental results of the pressure-dependent PL emission lines present three different types of pressure responses corresponding to a redshift (negative pressure coefficient), a blueshift (positive pressure coefficient), or even a sign change from negative to positive. Density functional theory calculations indicate the existence of competition between the intralayer and interlayer interaction contributions, which leads to the different pressure-dependent behaviors of the PL peak shift.

Effects of High-Energy Electron Irradiation on Quantum Emitters in Hexagonal Boron Nitride.

Hexagonal boron nitride (hBN) mono and multilayers are promising hosts for room-temperature single photon emitters (SPEs). In this work we explore high-energy (∼MeV) electron irradiation as a means to generate stable SPEs in hBN. We investigate four types of exfoliated hBN flakes-namely, high-purity multilayers, isotopically pure hBN, carbon-rich hBN multilayers and monolayered material-and find that electron irradiation increases emitter concentrations dramatically in all samples. Furthermore, the engineered emitters are located throughout hBN flakes (not only at flake edges or grain boundaries) and do not require activation by high-temperature annealing of the host material after electron exposure. Our results provide important insights into controlled formation of hBN SPEs and may aid in identification of their crystallographic origin.